The early 19th Century was the era of the great sailing ships and the sailors on these vessels sang sea chanties, both as work songs and as songs for entertainment. Since America was a brand new country, most of these songs are of English origin, but Shenandoah is uniquely American.
A capstan chanty, it was generally sung as members of the crew pushed the massive capstan bars around to lift the heavy anchor, a job that required a steady, continuous pull.
The song probably had its origins as a land ballad in the upper areas of the Mississippi and Missouri rivers and most likely was taken down to the sea by sailors who traveled up river looking for supplies to replenish their ships.
The original song was possibly about an Oneida Indian chief named Shenandoah. The song, however in the folk tradition, has been titled Shannadore, Old Mizzo, The Wide Missouri and numerous other variants on the two titles. It was also sung by the U.S. Army Cavalry as The Wild Mizzourye.